This invention relates specifically to instrumentation enabling methods for in-process, automated, continuous analysis of fluid systems for such as contamination, specified concentration of specific species, and the like. In the present application analysis of liquid baths containing processing solutions used in front-end processing in semiconductor manufacturing is used as a specific example, but the uses of the instrument are not limited to this exemplary application. The method and apparatus of the present invention are applicable in many areas, such as environmental, medical, pharmaceutical, industrial processing and other related and unrelated fields.
In a preferred embodiment the system of the invention uses a modified form of Isotope Dilution Mass Spectrometry (IDMS), known to the inventors as Speciated Isotope Dilution Mass Spectrometry (SIDMS). The exemplary method is an elemental and speciation threshold measurement method that is optimized for quality assurance at and near instrumental detection limits. The threshold measurement method is automated for unattended operation, and describes an In-process, Atmospheric Pressure Ionizer, Mass Spectrometer (IP-API-MS). The IP-API-MS apparatus is designed for identification and quantification of elemental contaminants or compounds and species in fluids, and in the exemplary case, liquid solutions.
Mass Spectrometry instrumentation is frequently used as the technique of choice in measuring parts-per-billion (ppb) and sub-ppb levels of elements and compounds in aqueous and other solutions of other solvents and reagents, as well as in gases. However, mass spectrometers typically must be operated and regularly calibrated by experienced technicians. There are, however, a wide variety of applications wherein continuous and unattended operation of the Mass Spectrometer would be highly desirable. These cases include remote operation, around the clock monitoring, and operation either in hostile environments, or where human interaction must be minimized, among others.
One good example of an application wherein around-the-clock, unattended monitoring is highly desirable is that of contamination-monitoring and control for wet-bath processing in manufacturing in the semiconductor industry. There are a variety of processes in semiconductor manufacturing wherein wafers in process must be immersed and treated in various ways, such as cleaning, neutralizing, etching and the like.
In the example of semiconductor manufacturing, various types of contamination are a critical issue; in particular contamination by particulate matter, as even very tiny particles may be larger than device and connection geometry in various stages of manufacture. At least partly for this reason a rigid clean-room environment is absolutely necessary, and minimal human interaction is highly desirable. Because of this, development and implementation of real time, in-situ sensors into clean room processes is considered by many to be one of the top five defect reduction challenges in the future of the industry. For further information regarding defect reduction in this industry, the article from International Technology Roadmap for Semiconductors 1999 Edition: Defect Reduction, Sematech, Austin Tex., 1999, (pg. 270), is incorporated herein by reference. There are many other references supporting this need, which is notoriously well-known in the art.
In order to accomplish unattended operation for wet bath contamination and constituent monitoring, apparatus must be developed that will automatically monitor constituent concentrations at their threshold level, accurately and without the need to compensate for inevitable systematic errors associated with instrument drift. Quantitation of concentration may then be obtained without the need for traditional calibration once the threshold level has been exceeded.
Prior art calibration techniques typically use calibration standards to generate a calibration curve relating instrument response to concentration of standards. Such a calibration curve is used to determine the concentration of unknown samples. A typical calibration curve of this sort is illustrated as FIG. 1 in this specification (curve A). Prior art techniques do not yield accurate results if the instrument response drifts or there is a response shift caused by a difference in the matrices between a standard and a sample. Mass spectrometers, typically used in such measurements, are especially susceptible to rapid drift, causing a change in the calibration response as shown in FIG. 1 (curve B). Such rapid drift results in a need for frequent calibrations that are normally performed by experienced technicians.
In calibration procedures, a considerable effort must be made to match the matrices of the sample and standard, to ensure ionization efficiencies, ionization suppression, or enhancements remain constant between a sample and the standard. Viscosity differences between the sample and the standard matrices may also cause unequal instrument responses associated, for example, with changing sample introduction rates, and are inevitable in real-world situations. Matrix effects altering solution viscosity or ionization efficiency can result in calibration changes such as shown in FIG. 1 (curve C). Clearly, in order to achieve full-time, real-time, measurement without human intervention for calibration of maintenance, a method and apparatus must be provided that overcomes these prior-art requirements.
The present inventors have chosen an enhancement of a known technique for achieving calibration-free mass spectrometry measurements. The known technique is Isotope Dilution Mass Spectrometry, hereinafter IDMS. In IDMS, one takes advantage of well-known, naturally-occurring isotope ratios in essentially all elements. For example; it is well-known that the average atomic mass of copper (Cu) in the natural state is 63.546, consisting of isotopes 62.9396 at 69.2% and 64.9276 at 30.8%. Very generally speaking, assuming an analysis for Cu content is required, one takes a sample of the solution to be analyzed, spikes the sample with an enriched standard solution having a substantially different isotope ratio than the naturally-occurring ratio, introduces the spiked sample to a mass spectrometer, and records the measured mass ratio between the isotopes. The measurement is going to differ from the naturally-occurring ratio and the standard spike ratio, and from the measured ratio, knowing the quantity of the original sample, and the quantity of the spike solution, one can calculate the single unknown, that being the concentration of Cu in the original sample.
IDMS, in a form known as Speciated Isotope Dilution Mass Spectrometry, hereinafter SIDMS, is a patented technique, described in enabling detail in U.S. Pat. No. 5,414,259, issued May 9, 1995 to inventor Howard M. Kingston Ph.D, and incorporated herein in its entirety by reference. The teaching in the referenced patent describes both the general (previously known) and the special (patented) technique as adapted for the instrument in embodiments of the present invention.
There are a considerable number of other available references describing IDMS, among them:                Fassett, J. D., Paulsen, P. J. Isotope-dilution mass spectrometry for accurate elemental analysis, Anal. Chem. (1989) 61 643A–649A.        Rottmann, L., Heumann, K. G., Development of an on-line Isotope Dilution Technique with HPLC/ICP-MS for the accurate determination of elemental species. Fresenius J. Anal. Chem., (1994) 350 221–227.        Heumann, K. G., Rottmann, L., Vogl, J., Elemental Speciation with Liquid Chromatography-Inductively Coupled Plasma Isotope Dilution Mass Spectrometry. J. Anal. Atom. Spectro. (1994) 9 1351–1355.        Fassett, J. D. and Kingston, H. M., Determination of Nanogram Quantities of Vanadium in Biological Material by Isotope Dilution Thermal Ionization Mass Spectrometry With Ion Counting Detection, Anal. Chem., (1985) 57 2474–2478,        
all of which are incorporated herein in their entirety by reference.
It will be apparent to the skilled artisan that employing the patented techniques of the above-referenced patent to Kingston is an excellent first step in accomplishing full-time, real-time mass spectrometry analysis of fluid systems, but that there are many other challenges in sample collection, sample handling, sample spiking, dilution, control, and many other areas to accomplish such a robust measurement and control system in real applications, such as in wet-bat analysis in semiconductor manufacturing, which has been selected as an exemplary application for describing the apparatus and methods of the present invention.
What is clearly needed is a modular system for deployment in real-world applications, which provides for all of the logistics of sample collection, flow-rate manipulation with extreme accuracy, sample alteration and preparation, extremely accurate spiking, sample introduction to analytical systems (mass spectrometry), information gathering and communication, on-line cleaning and purging between sample collection, and overall control among other things; and it is to apparatus and methods for accomplishing these ends reliably and efficiently that the following enabling descriptions of such a system is devoted.